Convert Checksum test to spec.

Library/Homebrew/test/checksum_spec.rb

+require "checksum"
+
+describe Checksum do
+  describe "#empty?" do
+    subject { described_class.new(:sha256, "") }
+    it { is_expected.to be_empty }
+  end
+
+  describe "#==" do
+    subject { described_class.new(:sha256, TEST_SHA256) }
+    let(:other) { described_class.new(:sha256, TEST_SHA256) }
+    let(:other_reversed) { described_class.new(:sha256, TEST_SHA256.reverse) }
+    let(:other_sha1) { described_class.new(:sha1, TEST_SHA1) }
+
+    it { is_expected.to eq(other) }
+    it { is_expected.not_to eq(other_reversed) }
+    it { is_expected.not_to eq(other_sha1) }
+  end
+end

Library/Homebrew/test/checksum_test.rb

-require "testing_env"
-require "checksum"
-
-class ChecksumTests < Homebrew::TestCase
-  def test_empty?
-    assert_empty Checksum.new(:sha256, "")
-  end
-
-  def test_equality
-    a = Checksum.new(:sha256, TEST_SHA256)
-    b = Checksum.new(:sha256, TEST_SHA256)
-    assert_equal a, b
-
-    a = Checksum.new(:sha256, TEST_SHA256)
-    b = Checksum.new(:sha256, TEST_SHA256.reverse)
-    refute_equal a, b
-
-    a = Checksum.new(:sha1, TEST_SHA1)
-    b = Checksum.new(:sha256, TEST_SHA256)
-    refute_equal a, b
-  end
-end